Add -Wmaybe-uninitialized pragma to union storage constructors

GCC 7+ produces false-positive -Wmaybe-uninitialized warnings at -O3
when deeply inlining through union constructors that initialize one
member while another (dummy_) exists. This is the same class of false
positive addressed in boostorg/variant2#55.

The pragmas are placed around the forwarding constructors in both
constexpr_union_storage_t and fallback_union_storage_t.

Author: alandefreitas

include/boost/optional/detail/union_optional.hpp

@@ -74,9 +74,20 @@ union constexpr_union_storage_t
 
     constexpr constexpr_union_storage_t( trivial_init_t ) noexcept : dummy_() {};
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 7)
+// false positive, see https://github.com/boostorg/variant2/issues/55,
+//   https://github.com/boostorg/url/issues/979
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
     template <class... Args>
     constexpr constexpr_union_storage_t( Args&&... args ) : value_(forward_<Args>(args)...) {}
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 7)
+# pragma GCC diagnostic pop
+#endif
+
     //~constexpr_union_storage_t() = default; // No need to destroy a trivially-destructible type
 };
 
@@ -88,9 +99,20 @@ union fallback_union_storage_t
 
   constexpr fallback_union_storage_t( trivial_init_t ) noexcept : dummy_() {};
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 7)
+// false positive, see https://github.com/boostorg/variant2/issues/55,
+//   https://github.com/boostorg/url/issues/979
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
   template <class... Args>
   constexpr fallback_union_storage_t( Args&&... args ) : value_(forward_<Args>(args)...) {}
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 7)
+# pragma GCC diagnostic pop
+#endif
+
   ~fallback_union_storage_t(){} // My owner will destroy the `T` if needed.
                                 // Cannot default in a union with nontrivial `T`.
 };